The purpose of a home theatre system is to reproduce a sound environment similar to that of a movie theater, that is to say, to reproduce a sense of depth due to the way the sounds are emitted. There are a number of techniques to achieve this result. These techniques are based on the presence of several loudspeakers placed around the room.
Notations such as “2.1”, “5.1”, “6.1”, “7.1” identify the number of loudspeakers used to reproduce the audio sequence. A “7.1” system, describes an eight channel sound reproduction system, of seven loudspeakers and bass and sub-bass loudspeaker commonly known as a “subwoofer” (the “0.1”). The seven loudspeakers are arranged as follows: the two front loudspeakers are used to produce ambient sound and music, the center loudspeaker, placed front is used to produce dialogue, the two side loudspeakers and the two rear loudspeakers are used to produce ambiophonic sound effects (“surround sound”). Meanwhile, the subwoofer is used to produce bass and sub-bass sound. A Dolby™ Surround 7.1 system is an example of this type of home theater system.
The home theater system normally includes an amplifier (Stereo, Dolby™ Digital, LC Concept, Digital Theater System (DTS), Sony Dynamic Digital Sound (SDDS), Dolby™ Pro Logic, Dolby™ Pro Logic 2, etc.). The image source may be, for example, a digital television satellite receiver, a digital terrestrial television (“DTT”) receiver, Video Cassette Player (VHS (Video Home System), SVHS (Super VHS) or digital hard disk drive, etc.), a CD player (Compact Disc), a DVD player (Digital Video Disc) (standard, Blu-ray, etc.) DIVX™ player, a multimedia personal computer or HTPC (Home Theater Personal Computer), a high definition movie player, game console, etc.
Images can be displayed on an HDTV, plasma screen, LCD (Liquid Crystal Display) screen, video projector, rear projection screen, etc.
Various technologies have been developed over the years to produce a home theater sound environment.
Dolby™ Surround Prologic (“DPL”) processing technology was invented by Jim Fosgate then sold to Dolby™ Laboratories. This was the first processing technology to give real depth to the soundscape. Sound is split into four channels: front left, center, right front and two back surround speakers, one left and one right, the surround speakers are identical.
The Dolby™ Digital (“DD”) processing technology was invented by Dolby™ Laboratories. This processing technology splits the sound into six channels (5+1): front left, center, front right, back left, back right and a channel for the sub-bass (to a subwoofer). The rear channels, or surrounds, are different and reproduce the entire sound spectrum.
The Digital Theater System (“DTS”) technology from Digital Theater Systems Inc., distributes the sound via six channels (5+1) the same as Dolby™ Digital, but the sound is less compressed and digital processing is different.
Dolby™ Surround Prologic I (“DPL-1”) is an improved version of Dolby™ Surround. Four channels are recreated from a stereo source. A center channel is added. DPL-I limits high frequencies in the surround channel to 9,000 hertz.
Dolby™ Surround Prologic II (“DPL-II”) processing technology was invented by Jim Fosgate then sold to Dolby™ Laboratories. This processing technology uses the same principles as DD and DTS but with different signal processing (two channels only).
Dolby™ Surround processing technology extracts three channels from two stereo channels: the front right, front left and rear surround channel are split across two loudspeakers which reproduce the same sound.
Dolby™ Surround Prologic II technology provides two additional channels to Dolby™ Surround Prologic: two different surround channels and a subwoofer channel. In addition to this, the surround channels broadcast the entire bandwidth, with no limitation at high frequencies.
Dolby™ Digital and DTS are developed by different laboratories and do not use the same algorithms or the same compression ratios. DTS uses a lower compression ratio than Dolby™ Digital.
Other formats have emerged alongside high definition (HD) technology, such as Dolby™ Digital Plus and DTS HD.
All the loudspeakers mentioned above serve a specific purpose. The two front loudspeakers reproduce the music and ambient sound, the center loudspeaker is for dialogue and the two surround (or lateral) loudspeakers give the effect of depth.
When five loudspeakers and a subwoofer are used (in the “5.1” version), Dolby™ Laboratories suggest that the back loudspeakers (left and right) are directed towards the listening position. In theory, the subwoofer can be placed anywhere, as the sub-bass sound waves it produces cannot be located by the human ear.
Technologies using seven loudspeakers and a subwoofer involve a doubling of the back center channel. These back center channels correspond to the two “back surround” outputs, the back left and right loudspeakers are simply the left and right “surround” positions.
Despite technological advances in the field of home theater systems, the fact remains that these systems generally require a room of adequate size to be able to position the loudspeakers correctly, a room that is not always available. Moreover, for current home theater systems, it is usually necessary to adjust the sound to quite a high level to reproduce all sound effects. However, the noise level required can become annoying when the room in which the system is installed is close to the neighbors. A new system mitigating these disadvantages would therefore be useful.